1. Technical Field of the Invention
The present invention relates to the oxidation, with oxygen or an oxygen-containing gas, of hydrocarbons and/or alcohols and/or ketones into the corresponding carboxylic acids.
2. Description of the Prior Art
The direct O.sub.2 oxidation of hydrocarbons, more particularly cycloalkanes, in the presence of a catalyst, is a process which has long been known to this art. The reason for this is that there are many obvious advantages in avoiding the use of an oxidizing agent such as nitric acid, currently in industrial favor, thereby avoiding the treatment of the nitrogen oxides generated.
In the many variants of such a process of catalytic oxidation by means of oxygen, cobalt is the catalyst most frequently of choice.
Thus, U.S. Pat. No. 2,223,493 describes the oxidation of cyclic hydrocarbons into the corresponding diacids, in a liquid phase generally containing acetic acid, at a temperature of at least 60.degree. C., using a gas containing oxygen and in the presence of an oxidation catalyst such as a cobalt compound.
U.S. Pat. No. 3,987,100 describes a process for preparing cyclohexanone and cyclohexanol by reacting cyclohexane with oxygen, in at least three successive steps, at a temperature of 130.degree. C. to 180.degree. C. and under a pressure of about 4 to 25 bar and in the presence of very small amounts of a catalytic system comprising cobalt and chromium. This process is carried out in the absence solvent and does not permit adipic acid to be prepared directly.
GB-A-1,086,951 describes the combined oxidation of mixtures of cyclohexanol and an aldehyde, in the liquid phase, with oxygen, in the absence or presence of a catalyst comprising at least one transition metal compound. This process produces cyclohexanone, epsilon-caprolactone and dicarboxylic acids starting from cyclohexanol, as well as the monocarboxylic acid corresponding to the aldehyde employed.
U.S. Pat. No. 4,902,827 describes an improvement in the air-oxidation of cyclohexane into adipic acid, in a liquid phase containing acetic acid, at a temperature of 80.degree. C. to 160.degree. C. and in the presence of an oxidation catalyst comprising a soluble cobalt compound and a soluble zirconium and/or hafnium compound.
And, more recently, EP-A-0,694,333 describes a catalyst comprising a cobalt salt and a ferric salt for the oxidation of hydrocarbons with oxygen.
Nonetheless, the selectivities attained employing the aforesaid prior art catalytic systems remain inadequate.